The invention relates to a system for time validation, comprising a terminal with means for tuning in to a number of different carrier frequencies, an authorisation device, e.g. a smart card, capable of communicating with the terminal and means to transmit time stamps, using a modulated signal having a carrier frequency, to the terminal. The invention also relates to a terminal and to an authorisation device in such a system. The invention further relates to a computer program for loading onto a programmable device e.g. a smart card, in order to use it as an authorisation device in such a system.
When a user of the terminal in such a system is only privileged to make use of it at certain times, a representation of the current time in the terminal must be validated to check whether the current time is such a certain time. Some systems keep time using only the time stamps. Others can comprise a clock in the terminal in order to keep the time. The time according to clock and time stamps can be validated by checking the two values against each other.
Existing systems for time validation are, however, generally not reliable. In order to keep the complexity and costs of such systems down, only the authorisation device is usually made secure against tampering. Terminals are usually of a standard type, not made to withstand hacking. A clock in the terminal could be set to a different time or perhaps slowed down or speeded up. Time stamps transmitted to the terminal can be recorded and played back later in a ‘spoofing’ attack.